The present invention relates to apparatus wherein the leakage flux from the welded seam of a base metal magnetized with an alternating flux is detected to detect the center of the welded seam, and more particularly the invention relates to an apparatus for detecting the center of the welded seam of a steel material, such as the welded joint of an annealed electric welded pipe which is less different in material properties from the base metal portion.
To detect the center of the welded seam of a steel material is essential, as for example, for effecting the desired noncontact copying control of a flaw detector in the ultrasonic flaw detection of a welded joint. In the past, with the flaw detection of the welded joint of a welded steel pipe by means of an ultrasonic flaw detector, for example, in order that the probe of the flaw detector may be controlled in a noncontact manner to follow along the center of the welded seam, the difference in properties, particularly magnetic properties between the base metal portion and the welded joint of the steel pipe is detected by the eddy-current detecting probe and a signal detecting the position of the welded seam is generated for copying control purposes. The use of such eddy-current detecting probe has the disadvantage of deteriorated detection sensitivity due to the mere detection of the eddy current induced in the steel pipe by the probe coil and moreover, excepting the cases where there is any large difference in properties between the base metal portion and the welded joint, it is not possible to effectively detect the center of the welded seam of materials, such as an annealed electric welded pipe in which practically all such differences in properties have been eliminated.